The color blood flow imaging technology, the most extraordinary and important function of a commercial color ultrasonic apparatus, is used to measure the presence of a blood flow in the human body and estimate the kinetic parameters of the blood flow in the human body. A schematic block diagram of the commercial color ultrasonic apparatus is shown in FIG. 1. A pulse signal transmitted from the probe enters the human body and, having been reflected by the human body tissue, the blood flow and the moving organs, is received by an ultrasound probe, and finally gets amplified, analog-to-digital converted and beamformed by an RF processing circuit to form a radio frequency (RF) signal. The RF signal may form a black and white image of the human body tissue through an envelope detection channel or may form a color image regarding the human body blood flow motion parameters through a color blood flow processing channel. The image is then sent to the display for displaying having gone through a merge of an anatomic image B and a color flow image C, a coordinate transformation, an image post-processing, etc., as shown in FIG. 1. These processes may be in different orders, or alternatively include additional image post-processing steps.
There is a key step going on in the color blood flow processing channel, i.e., a color frame averaging step, also known as color image time averaging processing. The object of this processing is to increase the signal-to-noise ratio (SNR) by time accumulation, so as to improve the sensitivity of the commercial ultrasonic apparatus in detection of a weak blood flow signal.
Color frame averaging processing technologies have been described in some public literatures and patents. Franklin [1] et al. of ATL proposed a color frame averaging method in 1993. This color frame averaging technology uses a first-order IIR filter for the frame averaging between consecutive color velocity frames. When the velocity increases, the velocity after the frame averaging is increased therewith swiftly; and when the velocity declines, the velocity after the frame averaging is decreased slowly. Thus, even during the end diastole, the color velocity can be maintained for a period of time to improve the color sensitivity. However, this patent does not take into account the possibility that there may be two consecutive frames, the directions of which are reverse. The color velocity frame averaging as described in document [1] is referred to as scheme I and described as follows. If the input blood flow velocity of the current frame is larger than the output blood flow velocity of the previous frame, the input blood flow velocity of the current frame is outputted as the output blood flow velocity of the current frame. Otherwise, the output blood flow velocity of the previous frame and the input blood flow velocity of the current frame are frame-averaged, i.e., Vout(n)=αVout(n−1)+(1−α)Vin(n), where, α is the frame averaging coefficient, Vin(n) refers to the input blood flow velocity of the current frame, Vout(n) and Vout(n−1) refer to respectively the current frame velocity and the previous frame velocity outputted by the color frame averaging.
Collaris et al. [2] described a time averaging scheme using a so-called “persistence filter”. This scheme also makes use of the IIR filter and moreover takes the possibility of opposing blood flow velocities into account. When the blood flow velocity of the current frame is in a different direction than that of the previous frame, or when the blood flow velocity value of the current frame is larger than that of the previous frame, the velocity direction of the current frame is used, and the blood flow velocity value of the current frame outputted by the color frame averaging processing is independent of the blood flow velocity value of the previous frame. Otherwise, a first-order IIR recursive filter is used to maintain the color blood flow velocity for a period of time. In contrast to scheme I, the scheme in [2], hereinafter referred to as scheme II, further considers the change in the directions of the blood flows between consecutive velocity frames. This is how scheme II is implemented: if the input blood flow velocity of the current frame is larger than the output blood flow velocity of the previous frame, or when the direction of the input blood flow of the current frame is opposite to that of the output blood flow of the previous frame, the input blood flow velocity of the current frame is outputted as the output blood flow velocity of the current frame. Otherwise, the output blood flow velocity of the previous frame and the input blood flow velocity of the current frame are frame-averaged, i.e., Vout(n)=αVout(n−1)+(1−α)Vin(n), where α, Vin(n), Vout(n) and Vout(n−1) have the same meanings as in scheme I.
In the literatures and patents published later on, some extensions are made to scheme II. To increase the pulsation of the blood flow, Forestieri [3] and Smith [4] adjust the magnitude of the frame averaging coefficient based on the calculated blood flow velocities of the current frame and the previous frame. Unfortunately, the final frame averaging algorithm does not take velocity aliasing into account. Likewise, Wong [5] of Siemens merely introduces two more parameters on the basis of scheme II, i.e., energy threshold and velocity threshold, to identify some irregularities occurring during the blood flow velocity calculation. Wong [5] of Siemens still disregards the effect of the blood flow velocity aliasing upon the velocity frame averaging.
All of these improved frame averaging techniques make no substantial changes to scheme II, though they consider the change in the directions of the blood flows between consecutive velocity frames. Moreover, a so-called “shadow” phenomenon always occurs in the clinical diagnosis of these schemes.
As apparent, the disadvantage of the existing color velocity frame averaging techniques is that when two consecutive frames have blood flows of opposing directions, the current blood flow frame is output directly without taking into account the relationship between the previous frame velocity value and the current frame velocity value if the velocity aliasing occurs. This operation of directly dropping the previous frame velocity value is a nonlinear processing, causing a “shadow” in the blood flow velocity diagram of the subsequent frame, which continues to appear in the following frames.
The cause for the presence of “shadow” is described as follows.
It is assumed that color scale indication bars are used to indicate the color blood flow velocity as shown in FIG. 2. The color red indicates a blood flow velocity in the direction towards the probe. As the velocity increases, the color scale exhibits black, dark red, red and bright red successively. The color blue denotes a blood flow velocity in the direction away from the probe. As the departure velocity increases, the color scale exhibits black, dark blue, blue and bright blue successively.
FIG. 3 shows two blood flow velocity diagrams, taking the carotid as an example, in which, FIG. 3a shows the blood flow velocity diagram of the previous frame having been subjected to frame averaging, and FIG. 3b shows the blood flow velocity diagram of the current frame under an auto-correlation estimation. The changes in velocity with time as characterized in these two diagrams are consistent with the actual changes of the blood flow velocity in the carotid. It can be seen from FIG. 3a that it is during the systole of the cardiac cycle that the blood flow velocity of the carotid is being scanned. Therefore, the carotid has a maximum blood flow velocity in the whole cardiac cycle. At this time, the blood flow velocity appears bright blue in the middle of the blood vessel, as indicated in area A in FIG. 3a. In other spatial areas of the blood vessel, such as area B shown in FIG. 3a, the blood flow is at a slightly lower speed than the blood flow in the middle of the blood vessel, i.e., the velocity in area B in FIG. 3a is slightly lower than that in area A, and the blood flow velocity in area B exhibits bright red. In the subsequent frame of the blood flow, as the blood flow velocity scanned at this moment is not the maximum blood flow velocity in the whole cardiac cycle, the blood flow velocity in the entire blood vessel exhibits dark red (or red), as shown in FIG. 3b. Both area A and area B in FIG. 3a exhibit dark red at this time.
FIG. 4 shows changes of the blood flow velocity with time in area A and area B during scanning two consecutive frames, in which PRF refers to a pulse repetition frequency. FIG. 4a shows the changes in the blood flow velocity with time in area A. During scanning the previous frame, the blood flow velocity in area A is just within the systole, when aliasing occurs to the blood flow velocity in area A, exhibiting bright blue. The blood flow velocity in area B is also within the systole, but the blood flow velocity is slightly lower than that in the middle of the blood vessel, thus exhibiting bright red. When scanning the current frame, as indicated by the vertical heavy dotted line on the right-hand side of FIG. 4, since the blood flow velocity being scanned does not correspond to the maximum blood flow velocity, neither the blood flow velocity in area A nor the blood flow velocity in area B is large, exhibiting dark red in the image.
According to the existing color frame averaging technology described in scheme II, in area A, since the blood flow velocity of the previous frame is bright blue, but the blood flow velocity of the current frame is dark red, the blood flow velocities have opposing directions. According to the frame averaging described as scheme II, it is the velocity of the current velocity frame that should be outputted, i.e., dark red. However, in area B, because the blood flow velocity of the previous frame and that of the current frame have the same direction, and meanwhile the blood flow velocity of the previous frame is larger than that of the current frame, according to scheme II, the output color blood flow velocity of the current frame is obtained by the recursion of the output blood flow velocity of the previous frame onto the input blood flow velocity of the current frame. Therefore, the blood flow velocity at this time is red instead of dark red. The flow of the frame averaging may be seen from the diamonds indicating velocities as shown in FIG. 5. The dot-and-dash lines in FIG. 5a and 5b each indicate the changes in the blood flows of the previous frame and the current frame after being processed by the existing color frame averaging technology. It can be seen from FIG. 5 that in area A, the output blood flow velocity of the current frame remains dark red, while in area B, the color of the output blood flow velocity of the current frame is lightened by the bright red color of the output blood flow velocity of the previous frame, thus exhibiting red. Thus, the prior art will result in a color blood flow velocity diagram shown in FIG. 6, in which the blood flow velocity in the middle of the blood vessel appears dark red, and appears red in both sides of the blood vessel. As a result, a dark red block stands amidst a red blood flow. This block of blood flow having a different color is referred to as a “shadow”. This phenomenon does not make sense in clinics. In a clinical blood flow velocity diagram, it should be that the blood flow velocity is high in the middle of the blood vessel, and the blood flow velocity at the proximity of the vessel wall is relatively lower. Furthermore, the larger the frame averaging coefficient, the greater the difference between the blood flow velocities in the middle of the blood vessel and in the proximity of the vessel wall; the greater the difference between the blood flow velocities of the current frame and of the previous frame, the greater the difference between the blood flow velocities in the middle of the blood vessel and in the proximity of the vessel wall. The same applies to the reverse blood flow. That is, the prior art would output a blood flow velocity diagram in which a dark blue block appears amidst a blue blood flow. Particularly, an obvious line trail will appear along the edges of area A and area B, i.e., the edge intersecting the dark red area and the red area, or the edge intersecting the dark blue area and the blue area.
These “shadows” and line trails are unfavorable in the color blood flow image. Therefore, there exists a need to provide a method and device that alleviates or eliminates these “shadows” and line trails concerning blood flow velocities.